I’ve always heard one of the reasons diets don’t work is, because, any time you restrict calorie consumption your body goes into starvation mode and lowers your BMR to conserve energy. Yet, there are very few times in your life, if any, when your body is completely free of breaking down and assimilating nutrients. Whether it’s food in your stomach or your colon, the body is constantly in the process of digestion. So, if this is the case, when is your body ever really starving?

Your body does not turn down BMR after only a day or two. You won't see that until well into true starvation at ketone bodies production begins to play a major role in energy supply. This really means that you have depleted all liver glycogen levels, gluconeogenesis is running significantly from glycerol-3-P, and levels of circulating glutamine are low. At this point in starvation, the body begins to signal with cortisol, glucagon, and other intercellular signals to slow down enzymatic activities.

The bodies activities really have to do with mass action and the availability of substrate. If the glycogenolysis released glucose dries up and lipid components like glycerol-3-P start going to blood glucose maintenance, then the body starts to sense those shifts. If amino acids start getting used for fuel and proteins have to be broken down for fuel, then by definition you are losing bodily functions. The loss of functions inside the human body are what cause BMR to decrease.

BMR does not decrease in food restriction diets that are done sensibly. This means that you never drop more than 1000 Calories from the diet at a time and never drop under 1200 Calories per day. I heavily recommend some form of exercise routine, but that is something that is another discussion.